Tuesday, December 29, 2015

A Single Schematic for the Simpleceiver!

Quite honestly I bristled a bit when I received several requests for a single overall schematic for the Simpleceiver project. Many of the inputs were if I had a single schematic I could better understand the project. Friends that is why we have the block diagram.

Well I guess my bent is if you don't understand the circuit blocks having the whole schematic may not lead to a "Level 5 Enlightenment". Another goal of the Simpleceiver was to encourage experimentation and the modules have been designed for the most part to enable matching to 50 Ohms. You can simply replace a Simpleceiver module with a different module and have a go at it so long as you look at the impedance match. An overall schematic may make that task more difficult.

But one kind soul DuWayne, KV4QB has taken on the chore to create such a schematic which is in a pdf format (Sorry guys no GIF's or jpg --so don't send me an email about the quality coming from a pdf) It is only through the generosity of KV4QB that you have the singular pdf. Also please no emails can you break the circuit up into pdf blocks!

I have repeatedly stated the use of LT Spice in developing and working with such a project. A serious homebrewer needs to adopt and adapt to that tool. If you want to make circuit changes the first thing you should not do is send Pete and email asking me to do the analysis. You have the tools and examples, so you the homebrewer need to ask the question of yourself about substitutions.

Click on this "LINK" and it will take you to the Simpleceiver Software link which is hosted on my website http://www.n6qw.com. There you will find the Simpleceiver Single Schematic in .pdf document.

Thursday, December 24, 2015

Revisions to the Simpleceiver Detector

We are delighted to see that there are homebrewer's out there who are building this project and that is most gratifying. This gives me a sense that all of the time I spend developing circuits and maintaining this blog is having some impact and benefit to the greater ham community.

In an exchange of emails with Bob, K6GGO, who built the product detector with the intent of having an 8.0 MHz IF, he reported that he physically built the circuit and ran some test to characterize the performance. His conclusion was that the circuit performed nicely but was seeing an output response capability that extended beyond 100 kHz and in fact had seen a peak response closer to 200 kHz.

My response to Bob was that I had seen the same thing but simply ignored anything beyond 20 kHz as this was AUDIO and anything beyond that would never be heard by humans --maybe your dog would hear it? So in the interest of those who would have sleepless nights knowing that the output from their product detector could support a frequency response output beyond the audio range, I did some further evaluations.

Using LT Spice it was a simple matter of making some trial runs to see the effect of changes. Basically I used the Direct Conversion Receiver to test the part change but the same change is made in the Product Detector circuit in he Superhet version.

I am happy to report there will be no output or peak response beyond the audio range and thus those who may suffer a sleepless night will have a peaceful rest. One component value change did the trick. L1 which is in the output filter network is changed from 1000 uHy to 100000 uHy. (For the old school guys like me that s 1 Milli Henry to 100 Milli Henry). I am delighted to report that 100 Milli Henry chokes may be acquired from Mouser 100MHy Choke The cost is a staggering 38 Cents.

There is much good in collaboration and having an openness to making changes. One part value change = peaceful sleep!

Monday, December 21, 2015

Thoughts on the Control Circuitry

We have all heard that a long journey must start with the first step ( I guess 22 parts qualifies this as a long journey). With the Holidays my bench time has been somewhat limited but I did want to provide some of my thoughts on a control circuit for the transreceiver in the event there are those who want to forge ahead.

Perhaps one of the most critical items to make the receiver and transmitter play as a unit is how to control the transition from receive to transmit AND to accomplish that end without hiccups, burps, spurious artifacts, hot switching, "kerchunks" and on an on. Those who have followed my projects will find that I like to recycle circuits from prior projects chiefly so I am not continuously trying to reinvent the wheel and secondly since they are proven their performance is known.

The solid state control circuit that you will see shortly was a result of a project I did in 2009 where everything was switched with relays. At that time the frequency control in the dual conversion transceiver (my solid state version of the Heathkit HW-100) was a PTO from a Ten Tec Triton IV. There was such a terrific back emf from all of the relays that the frequency would change by a few kHz every time there was a TR.

Below is the breadboard for that project. If you look in the upper portion of the photo near the center you can see a huge 4PDT Power Relay and somewhat obscured is the RF boards that had relay selected Band Pass Filters and another board that relay selected Low Pass Filters. There was also a TR relay to redirect the antenna to the right portion of the circuit. So after a week or so of trying to resolve this issue I came up with a solid state switching scheme which has been refined with several iterations and will most likely need work for the Simpletransreceiver BUT it is a place to start.

Next is the schematic of the baseline control circuit. STOP --I already see many of you sending me an email that since I have an Arduino why not use that to do all of the control. I am aware of that but that would require more development time. So why not use a proven hardware solution!

(Some day I will share my Arduino Linear Amp control scheme that senses SWR, Power Source Voltage, Over Voltage, Over Temperature and sequenced step start of the power supply --so I do know how to do it --but an overkill for this application.)

For those who may have seen my JABOM transceiver (Just A Bunch Of Modules), this was the circuit used for that project. Essentially the PTT triggers the optoisolator (4N35) to toggle the 7400 IC wired as an Inverter. So OK use your favorite inverter chip --this is what I had in the Junk Box! With the wiring as shown the + 12 VDC R is always "On". BUT when you key the PTT this toggles the SN74LS00 to the other state and there is a transition from +12 VDC R to +12 VDC T. So long as the PTT is held in --you have + 12 VDC T.

The TIP32C PNP devices are good for a couple of amps --if you want to switch bigger loads then circuits with power MOSFET's would be the order of the day. So please no emails that "your circuit doesn't work or why did you do this or that?" This is a starting place and the circuit WORKS! You are welcome to make any changes.

There have been embellishments of this same circuit where I used a NE555 timer that was keyed and with the inclusion of one relay added a timed closure of the PTT such as you would have while sending CW or for a Tune function. It was successfully used in my KWM-4 SSB/CW transceiver. There is no difference between using 1N914's or 1N4148's -- I am merely trying to head off a flood of inquiries! Also read the schematic and notes carefully -- I have received emails about what is the * after the SN74LS00 and what do you do with the unused pins -- the schematic is clear --GROUND THEM!

[Caution: This control circuit is a starting point to demonstrate a way to electronically switch most of the circuits from receive to transmit. It is envisioned that it will work pretty much as shown BUT not having been tested with the receiver and transmit boards and other ancillary equipment such tests may result in some modifications. Thus the material is being presented now to show that there is plan being set forth to control the two major elements. Since this is a transreceiver and not a transceiver per se there may be other additions required. Bottom line "Heads Up".]

I have not detailed the interconnection to the receiver or transmitter circuits --remember this is a starting place. But basically the plan is to identify modules that would always remain powered regardless of Transmit or Receive such as the Arduino/AD9850 and probably the Audio amp stage. What will be switched are circuits such as the Rx RF AMP and Rx IF AMP & Product Detector and a maybe on the BFO. On the transmit side the Microphone Amp and the transmitter specific circuits and final the antenna change over. In work is a block diagram of the control circuits but some of that will be refined after hookup.

This information is being provided now so that those who want to start the control circuits have most of the information needed. Below is the Low Pass Filter Information. The inductors have been wound using a standard number of turns and the cutoff made slightly beyond 8 MHz and the second harmonic suppression is almost unbelievable. In/Out is 50 Ohms.

Many are endeared to the BITX IRF510 Final amplifier --I am not and a future post will include a "real RF Device" final amplifier as used in the JABOM, ZIA and KWM-4 transceivers.

The J310 Companion Transmitter ~ The Journey Continues!

If it hasn't hit you right between the eyes, by now, then turn off your soldering iron. So far we have built what I consider a pretty decent receiver for not a lot of money. There is ongoing development work on that part of the project inclusive of the incorporation for the W7ZOI Hycas AGC. That addition is not quite ready for prime time; but we are close.

The building of that receiver involved what I call a common template, the J310's configured as a Dual Gate MOSFET. That has worked very well and you only need to listen to the you tube videos and you can adjudge for yourself. We also introduced the use of LT Spice as a simulation tool. Most of the time is spent at the computer and when we are ready to solder up a circuit it is pretty much the final configuration so there is minimum time wasted rebuilding hardware.

Now we are ready to proceed with the building of the transmitter portion of the transceiver. For those who tuned in late, the project involves building a separate receiver (done) and a separate transmitter with a sharing of the LO and BFO that commonly links the two to form a transceiver. Some would argue hey I can build a BITX and be done. Well for those so inclined have at it. But if you want the ability to fine tune the receiver and transmitter for optimum performance, including features not normally found in the BITX like AGC and maybe ultimately a color display then this project is for you.

There is much flexibility in my design --yes I can really say it is my design wherein you can employ different filters (both homebrew and commercial) as well as different IF frequencies. We have provide information for IF's such as 12.096, 9.0 and 8.0 MHz. The use of the LT Spice makes for short work in being able to do that. Below is the block diagram for the low level circuits for the companion transmitter. You will note that the very same circuits employed in the receiver are now used in the transmitter. So if you have built the receiver all you will have to do is to duplicate several of the circuit blocks and you are there. The one new block is the microphone amplifier which will be covered shortly.

This is our starting point and be advised there may be some alteration but it is a the first step. One possible change is to take the 2nd SBL-1 which is used between the IF amplifier and the Band Pass Filter and put another DGM stage in there wherein the J310's are configured as a mixer stage. That very likely will be the case but for now we have a placeholder with the 2nd SBL-1

The one block on the above diagram we have not seen before is the microphone amplifier and we will next provide that schematic --any similarity to the Product Detector template is not accidental!

WE have added the filter on the output so we have a very nice audio spectrum being presented to the Balanced Modulator. R3 is a 10K Trim Pot and the center wiper has a 10 NF capacitor (called C3) connected to it and the other end of the capacitor is connected to the Pins 3 & 4 on the SBL-1.

The audio generator used for the LT Spice simulation presumed a 600 Ohm series impedance and thus the amp is predicated being used with a 600 Ohm low impedance microphone which seems to be the norm today. While not included in the simulation this would be a great opportunity to test one of those inexpensive electret microphone cartridges. A bit of bias for the microphone and it would simply connect into the input.

Below is the output plot for the microphone amplifier as above. See the note about where to connect the 10 NF capacitor referred to as C3. This plot shows a very nice output curve. Thank You Pete and Thank You LT Spice.

We decided to publish this schematic since some of the experimenter's wanted to get a head start on the build of the transmitter.

Addendum:For those who are itching to build the transmitter below is the board layout that will enable you to get most of the modules on a single board. The squares are 2/10 of an inch so you can count the number of squares and figure the size ( about 4 X 6 inches). I have decided to build a specific BFO carrier oscillator for the transmitter stage. There is a good reason for this -- loading on the BFO when you shift between Tx and Rx most likely would result in a frequency change. I have stock of about 30 pieces of the 12.096 MHz crystals . I ran all 30 through the Simpleceiver and found 4 that are "dead on" with the crystal in the receiver --so we now have BFO crystals for the transmitter. Look at the arrows as that will be the signal flow. Virtually all interfaces are at 50 OHms so there will be many matching transformer --- get a large stock of FT-37-43's (Toroid King is a good place to find these. The BFO layout will be the same as is the IF amp block. The microphone amp block will be similar to the product detector.

Addendum: Schematic for the W7ZOI AGC Circuit.

Addendum #2: The Transmitter Block Diagram

In Part 19 we recapped the elements of the Direct Conversion Receiver and now in Part 20 we will move to the Superhet configuration. For the last week or so I have been listening to the Simpleceiver Superhet and there is a great deal of satisfaction to finally see all of the blocks and endless simulations come together. Cost wise there is about $60 in parts with the most expensive being the AD9850 followed by the LCD. I purchase parts in bulk and so my cost estimate may be skewed versus buying resistors one at a time. In summary --the sound and frequency stability outshines the $60 investment!

Let us start by looking at the Superhet block diagram as shown below. The darkened blocks are from the Direct Conversion Receiver. So to make the conversion only five additional elements need to be added plus you will need to reprogram the Arduino/AD9850 with the IF filter offset.

Four of those blocks (SBL-1, 1st and 2nd IF amp and the Filter) were built on a single board which is shown below and which I call the "Mainboard". The "island squares" are 2/10 of an inch so you can count the squares and get a feel of the size of the board. I used my CNC Mill to cut the squares but this could be made using the W1REX MePads and again count the squares and you are there. If you feel comfortable using nasty chemicals then the board could be built the old fashion way -- chemical etching. Or just do as I read in a recent GQRP SPRAT article, a ham used a wood chisel to cut his squares --Crude perhaps; but very Effective!

The final element to built following the Mainboard, will be the BFO. Being slightly conservative I used a single JFET versus the dual DGM approach. That is seen below and at the time I built the product detector I left space on the board for the BFO. Again count the squares (2/10 inch) and you get a feel for the size. The MePads will work nicely as will etching or use of the chisel. Look carefully at the Product Detector layout as this same approach was used on the mainboard! This keeps the leads short and direct! In place of the RF choke assembly in the product detector is the tuned network for the amplifiers and can utilize the same overall space requirements. Oh a bit of tribal knowledge here. Note that I made a crystal socket from a three hole SIP strip. This was done to enable finding the "just right" BFO crystal. I simply cycled about a half dozen crystals in the socket and by adjusting the small trimmer found the right one. Later you could remove the socket and permanently solder the crystal to the board

I did receive an email about "why did you use coax from the Product Detector to the input of the audio amp --grey colored cable"? That is not coax but small diameter microphone cable or at least that what the reel says.

Next are the schematics for the amplifier stages and BFO which in this design are at 12.096 MHz. I have also included a design for a 9.0 MHz version in the event that a builder does not want to undertake the somewhat arduous task of homebrewing a 12.096 MHz filter and choses to use either the INRAD Model #351 or the GQRP 9.0 MHz unit (and for that matter 9.0 MHz filters out of any radio).

All that is required is the proper matching transformer to go from 50 Ohms to the In/Out of the filter. For the INRAD it is 200 Ohms so a 4 turn to 8 Turn transformer on an FT-37-43 cores gives the 4:1 match. In the case of the GQRP it is 500 Ohms so you would need a 6 to 19 Turn match. If at this point you do not understand matching transformers and turns ratio squared -- you probably should not build this project as all interfaces are at 50 Ohms and that is achieved by liberal use of ferrite matching transformers!

The Schematic below is for the 12.096 MHz amplifier and includes a 2 dB resistive pad used only in the 1st IF amplifier stage and is omitted in the second stage. This pad is there to provide a constant load before going into the Crystal Filter. There is a notation about the matching transformer on the input side (Gate #1). The Four Pole Crystal Filter is 150 Ohms so a 3:1 match is needed. There are many combinations that put you quite close but a 4 turn to 7 Turn is probably the simplest and that is wound on the FT-37-43 Ferrite Core. (BTW a 8 to 14 turn transformer will do the same -- do you see a pattern here?) By design we repeat (but reversed) this transformer on the output side of the crystal filter so we have a 150 Ohm to 50 Ohm match.

On the output of the second IF amp there is a match of 50 Ohms to 2200 Ohms and that neatly is the input side of the Product Detector so no other matching transformer is needed --- 50 Ohms to 50 Ohms.

Note the 10K trimmer pot arranged as a variable resistor in series with a 3.3K resistor provides a convenient means of adjusting the amplifier gain.

For a 9.0 MHz IF amplifier only three parts are changed. L1 is now 6.985 uH and consists of 35 Turns of #24 on a T-68-2 core and C1,C2 are now 68 PF. The same output matching transformer would be used and the resistive pad is only used on the 1st IF amp stage. Below is a plot of the 9.0 MHz IF amplifier simulation. Kind of cool!

﻿

Next is the Crystal Filter and as was mentioned in a earlier posting there is a rigorous process to building a homebrew filter that starts with finding four crystals that have no more than a 50 Hz difference among any of the four. Nick Kennedy, WA5BDU has documented the information on the "how to do this process." I did not go through that total process but did select the 4 crystals that were closely matched. I guess if one has weak knees about building a filter and despite the dye in the wool hardcore build everything gang, there is absolutely no shame in having a commercial unit in your radio. The data has been presented to do this.

Finally we have the BFO stage and that is just something I found once and have repeatedly used it in projects.

﻿

Completed Simpleceiver

﻿

The sketch for the Arduino/AD9850 will be uploaded to my website on December 15, 2015 and can be found at http://www.n6qw.com The wiring for the Arduino and AD9850 is shown below.

That completes the basic receiver portion of the project although in work is an AGC circuit but that may have to wait until early 2016. After the Holidays I will start with a discussion of the Transmitter portion of the project.

Addendum ~ Below is the baseline W7ZOI AGC circuit that will be modified for use with the Simpleceiver. With our studies of how Gate #2 responded with changing bias voltage our modifications will include setting the "no AGC" to about 5 Volts and "full AGC" to less than 1 volt. Thus this is a Reverse AGC --stronger signal less positive voltage on Gate #2. This is an "IF Derived" AGC" and the sample point is a couple of turns on wire wound over the inductor of the 2nd IF amp stage and connected to points "X and Y". [Wind in the same direction.]

The output of the AGC can be applied to the RF Amp Stage as well as the 1st IF amp. Once I have it built and adjusted, then I can further evaluate the optimum configuration. I will also add in the S Meter using an Analog meter, although if a TFT color display were used it would be possible to include a bar type S Meter right on the display (been there and done that). The transistors are 2N3904 and 2N3906 and the didoes 1N4152. Another modification would be to make R6 a 500 Ohm trimpot connected as a variable resistor.

The 50 Ohm Output R3/R4 is ignored since we are merely sampling the signal. In the original hycas this output was sent to the product detector for which we have used a different method to do that.

Addendum #2: The Transmitter Block Diagram --look closely as most of it is already built!!!! Now you know the why of a common template!

Note: For those wanting to use an 8 MHz homebrew Crystal Filter IF frequency the values for the IF amplifiers are as follows; L1 = 34 Turns on a T-68-2 using #22 Wire andC1, C2 = 100 PF. Note you will have to determine the Z in/out of the 8.0 MHz filter and build the matching transformers accordingly -- most likely the match will be in the 150 to 300 Ohm range. A 4 to 8 turn will work for the 200 Ohm and a 4 to 10 turns will work for 50:300 Ohms (1:6 where 16:100 approximately 1to 6).

Thanks for riding along --this has been a wordy journey but hopefully informational, inspirational and just a plain lot of fun. Here is wishing the N6QW blog readers the very best of the Holiday Season and a Healthy and Happy New Year﻿

Tuesday, December 8, 2015

It is always good to take a step back to review where you have been to see where you need to go. The Simpleceiver project so far has taken us from a Direct Conversion Receiver to a fully functional Superhetrodyne Receiver with a crystal filter. Our next step will take us to building an accompanying transmitter so that the project will morph into a fully functional 40 Meter SSB transceiver. The project was broken into modules so that each module could be built and tested before moving on to the next stage.

A backbone to this project has been the extensive use of LT Spice and my attempt to provide a detailed documentation of virtually all aspects which hopefully has been useful. I must confess to receiving a bit of criticism to such an approach. An email read in part "stop the blabbing and just provide me a schematic and parts list. I know how to solder two wires together." So to that end I must duly apologize to others who have felt this way but perhaps have not vocalized such an input.

A word here about parts lists. I do not provide parts lists based on some bad experience I had with a publication of an article in QRP Quarterly. I was asked to provide a very detailed parts list for a project article using only two or three suppliers. That effort took longer than to design the radio, build it and then write the article. After publication I received an email asking where to buy two 10K 1/4 watt resistors. My response was to find a local Radio Shack and make that inquiry of them. The return email "Hey you didn't give me the Radio Shack Part Number". I did not respond to that email nor will I ever make a parts list again --sorry folks but You Do Have To Do Some Of The Work!!!!

In my approach to radio receiver construction, the first module to be built would be the audio amplifier stage. Earlier in Part 4, I presented three designs any of which will work. For the hardcore, dyed in the wool*, "I must homebrew everything or it is not a true homebrew radio", then the discrete component version would be your obvious choice. For those who want plenty of reserve audio and have no concerns about using black boxes (IC's) then the NE5534 driving the LM380 is the amplifier of choice. [* Hopefully I used the correct form as inputted to me by the pedantic observer.]

The current configuration of the Simpleceiver has the IC approach. By building this circuit first, those new to homebrewing can develop the skills necessary to proceed with the follow on modules. I am convinced that the discrete audio amplifier should be built once and then forgotten. With that one time build you will learn about every component and its function. But then get over it --you do not need to repeat that experience every time. Based on our experience with the Lets Build Something project --that circuit has too many parts, too many opportunities for screw ups and too little output for the parts invested.

Direct Conversion Receiver

The Direct Conversion Receiver use five modules including the audio amplifier (build first), the product detector employing the J310s configured as a Dual Gate MOSFET, the Local Oscillator (LO), the Band Pass Filter and finally, if needed, an RF amplifier. In the case of the LO my preference is to use the Arduino driving the AD9850 as this has many advantages including the LCD Display and the fact that this combination can be used as a signal generator for building other portions of the radio. The second and third choices in that order are the LC VFO and then a VXO. Below is our Block Diagram for the Direct Conversion Receiver.

For the hard core discrete component builders who abhor the use of such modern technologies like the AD9850 or Si5351 and will only use a conventional LC Oscillator or a Crystal VXO feel free to use your favorite deisgns. Many designs abound for LC Oscillators and quite frankly I will leave that to the builder. However I will provide a design for a crystal switched VXO that can be used with the Simpleceiver.

Mind you the simple VXO will not give full band coverage and in short order those who opted for this approach will most likely gravitate to the LC Oscillator or the Arduino Driving the AD9850. Before I receive a rash of emails about VXO's there is a problem with using 5.0 MHz crystals in a VXO --the amount of swing is proportional to the base frequency. So a few possibly up to 10 KHz is pretty normal. Using 15 MHz crystals in a VXO enables a far larger swing especially in a Super VXO configuration.

One prior trick I used for a wide frequency excursion was a crystal switched heterodyne VXO. This circuit involved an 2N3904 simple crystal oscillator operating with several different 6 MHz crystals that could be panel selected and an NE602 that used 12 MHz Crystals in a Super VXO on pins 6 & 7. The 2N3904 signal was fed into the NE602. The resultant mixed signals enabled about 120 KHz at 19.2 MHz for injection into a 20 Meter transceiver with a 4.9152 MHz IF. Caution you need a Band Pass filter following the NE602 so you only pick off the 19.2 MHz component! A simple panel mounted switched enable two 60 kHz slices with 6 MHz crystals. When using cost as a determinant in a decision as to which approach --you might find this hard to believe but they all cost about the same and that is around $20.

Another consideration for an LO is a heterodyne VTO (varactor tuned oscillator). The circuit below was employed in a 30 Meter CW transceiver I designed and was published in 2013 in a QRP Quarterly article. I was encouraged to write this article based on the fact that readers would love a 30M CW transceiver --well I had zero emails about how much this was loved BUT one thing did come from this article and that was how I piggy backed on another person's work on how to install RIT in an LO.

The circuit below has an RIT functionality that only works on Receive! Basically the VTO operates around 2 MHz and that is mixed in an NE602 with a fixed frequency crystal operating at 12.96 MHz. The IF was at 5.0 MHz. So the output of this VTO was actually at 15 MHz (12.96 + 2.14) and was above the incoming frequency at 10. MHz (30M) and the subtractive mix was 5 MHz --the IF. The diagram shows some constants for keeping the VTO range and by using a 10 MHz heterodyne crystal mixed with the 2.14 MHz VTO produces 12.14 MHz. The subtractive down mix is 5.0 MHz on 40M. There is a tuned network following the NE602 to assure output in the correct range. Somewhat with pride I think this was a bit of innovation on my part --BUT nobody gave a crap given the lack of interest in the project. Purposefully the VTO was chosen for a low frequency (2 to 3 MHz) as it is much easier to treat drift in the VTO. Regulated voltages are a must and you will note that there is not a singular capacitor in the VTO tank network --there are multiple caps so that this minimizes circuit heating in the capacitors and using high quality NPO caps minimizes an capacitance change with temperature. Boiling the inductor in water also puts some "magic mojo" to stress relieve some of the inductor properties.

[I have not modified this VTO to use with the Simpleceiver and these are merely thoughts you should have if you attempt to use this circuit!]

For the DCR we have to supply an LO directly in the 40 Meter band so that filter constants for 40 Meters are already there for you. But how do we get the output to be in the 7.0 MHz range? We have several choices and the first is to use a 9.216 MHz crystal (a standard value) since a subtractive mix is 9.216 - 2.14 and that gets you there picture perfect. Another option is to use the 10 MHz crystal and then move the VTO up to 3 MHz where the subtractive mix is 10 - 3 = 7. In either case you must use the 7 MHz tuned network on the output of the NE602.

If a builder would want to employ this approach for the Superhet then two things have to happen: 1) and crystal/VTO combination must result in a tunable 5 MHz output and the output network must be changed to 5.0 MHz. Just thinking ahead here --there is a sound reason to move the VTO to range 3.4 to 3.7 MHz and use an 8.5 MHz heterodyne crystal (a standard value). Here is why --the possibility of unwanted mixing products. with this range and because we are up mixing to the 12.096 MHz IF (keeping in mind this is tuning backwards -- the higher the LO frequency the lower the received frequency). With the VTO at 3.4 MHz the subtractive mix is 5.1 MHz and the up convert to 12.096 means a received frequency of 6.996 MHz and with the VTO at 3.7 MHz the upper received frequency is 7.296 MHz. The concern is that the VTO frequency would drop to 2.5 MHz as the second harmonic of that frequency is 5 MHz which could pass through the tuned network. At 3.4 the 2nd harmonic is 6.8 MHz and at 3.7 the 2nd harmonic is 7.4 Mhz which is also outside of the ham band. You must always use care when making frequency selections. Again this is presented solely as an idea piece and was not modified for use with the Simpleceiver

Now for you dye in the wool only use discrete component builders you should be salivating at this point! But that is a lot of hardware that could easily be done with an Arduino and a AD9850!

The following are the remaining schematics for the Direct Conversion Receiver:

In Part 19 we will recap all of the schematics for the Direct Conversion Receiver and Part 20 will be dedicated to the Heterodyne version. Mind you most of what is done with the DCR will find its way into Part 20.

Friday, December 4, 2015

More Detail on the Design and Alternatives in the Simpleceiver Receiver Project.

Addendum: 12/05/2015 40 Meter CW Band Pass FilterAddendum #2: 12/06/2015 "Mea Culpa" for the improper use of "die in the wool" (Crystal Filters)

Based on some inputs I have received, I wanted to explore several "details' of the design and also to explore the use of IF frequencies other than 12.096 MHz. So let us begin by looking at the "gain adjustment" features of the Simpleceiver design. Currently there are three stages which can manually have their gain adjusted which includes the RF Amplifier Stage and the two IF Amplifier stages. The next step of course would be to replace the manual gain controls with an AGC circuit.

The use of the Dual Gate MOSFET was not accidental; but by design since it is such a versatile circuit element in that this device can be used as a oscillator, amplifier, mixer or as a detector. We have used the J310's configured as a DGM for everyone of those applications. The other desirable feature is the ability to add gain control especially when configured as an amplifier and this will be the next discussion.

We will start with the RF Amplifier Stage. The circuit diagram for our 40M RF Amplifier is shown below. The original simulation for this circuit did not include the matching transformers at the input and output and as I noted transform the in/out of the stage to 50 Ohms. The gain adjust is accomplished with R1 and R. This was shown earlier and has caused some head scratching among those who viewed the schematics and my note of response to them: Spice does follow Ohm's Law! We will now look at more detail of the effect of the biasing.

We will now look at just the bias circuit which feed the Gate on J1 --this often is called Gate #2. The graphic shows a source of 12 Volts DC and there is a series of three resistors, starting with the 22K connected to a 10K Trimmer pot connected as a variable resistor which is in turn connected to a 2.2K resistor to ground.

See the Graphic below. As the trimmer pot is adjusted through its range the total series resistance changes from a maximum of 34.2K Ohms (10K fully open) to where the 10K is essentially shorted (=0) and the total series resistance is now 24.2K Ohms. By picking off the Gate 2 voltage at that point where the trimmer is adjusted through its range we have a voltage divider effect and using Ohms law we can see voltage will change there is a range of Gate #2 voltage goes from 4.28 Volts to 1.09 VDC. Yes Virginia there is a Santa Claus and Ohms law still applies.

But there is also an opportunity to look at the circuit and by inspection see the approximate voltage values that will result on Gate #2. This can be very useful as a sanity check on what you perhaps are measuring or expecting versus what may be actually happening.

Lets look at the case where the 10K is fully applied. Thus we essentially have 2/3 of the total resistance (22K) and 1/3 of the resistance (12.2K) at the voltage divider point. Thus the 12 volts should split roughly 8 volts and 4 volts. So we should expect somewhere around 4 volts at Gate #2 --the actual value is 4.28 Volts so our approximation was close!

Looking at the other case where the trimmer is shorted we now have 22K and 2.2K (how convenient --no accident) thus we have a 1 to 11 split or about 9% of the voltage appearing on Gate #2 -- 0.09 X 12 = 1.08 Volts and the actual answer is 1.09 --so again our approximation is very close. Learn to do this as it is an invaluable tool when you are troubleshooting a circuit --if there is something radically different being measured from your approximations then you have narrowed down the problem.

We now will look at the effect of these voltage changes on the gain of our RF amplifier stage. But before doing that I will share that as you drop the voltage on Gate #2 the stage gain is reduced (you will soon see that). But it is the direction of the voltage change/gain reduction that is important especially if the voltage change is accomplished using automatic gain control versus a pot adjustment. We have a case of what is called negative AGC versus positive AGC. By applying a lesser positive voltage 4.28 >1.09, the gain is reduced by 10 dB and any circuitry used for AGC control must have an output that as the input signal is stronger the resultant AGC has to be "negative" going.

I have reworked the RF Amplifier Simulation to reflect that being supplied to Gate #2 in one case is 4.28 VDC and in the second case 1.09 VDC.

Now we have changed the Gate #2 Bias to 1.09 VDC﻿

Just eyeballing the two curves we can see that a 3 Volt downward change (4.28 > 1.09) results in a 10 dB change in gain for the RF Amplifier. In the old days (W7ZOI Solid State Design for the Radio Amateur) a simple toggle switch to short out the 10K Pot could be a panel mounted RF Amp control switch.

We are looking to modify the W7ZOI Hycas AGC circuit for use with the Simpleceiver. I have done this previously on another SSB transceiver that used DGM's in the IF amp stages. Just need to build one for this radio. That will be the subject of a future post.

Other Crystal Filters.

Homebrewing a crystal filter was covered in an earlier post and for the most part is a lot of drudgery but the die in the wool homebrewer's will argue that using a packaged unit is not true homebrew and look what you will learn. The cost is another factor --pennies for the crystal versus tens of dollars for the commercial units. Well I have been there and done that and know how to do it --but it is hard to beat a commercial unit built on a line with exacting equipment. True there will be those who will post my homebrew is better than commercial and maybe it is -- but my time is limited and so an easier solution is a purchased unit. One of the easiest to find is the 9.0 MHz from INRAD Model #351 --about $30 and its in/out is 200 Ohms. But you will have to find the matching crystals. The GQRP club sells a 9.0 MHz unit and the crystals but there are additional costs for shipping and membership in the GQRP club. If you shift to the Si5351 versus the AD9850 then the crystals are a non-issue.

The critical area that will require modification to accept the 9.0 MHz Filter is the IF Amplifier stages and to some degree the BFO depending on whether you use crystals or the Si5351.

Below is the design for the 9 MHz IF amplifier. Note the biasing is the same as for the RF Amplifier and the same 10 dB change takes place with an adjustment through the range of the 10K Trimmer. On the input side the hookup involves a matching transformer consisting of a 3 Turn Primary and a 20 Turn Secondary wound on a FT-37-43 ferrite core. I used #26 wire. [3^2 = 9 and 20^2 = 400--- 400/9 = 1:44 which is the same ratio as 50 Ohms to 2.2K.]

On the output side there is another matching transformer to match to the input of the Crystal Filter. For the GQRP Filter the match is 50 Ohms to 500 Ohms or a 10:1 match. That is easily done with a 6 Turn primary and a 19 turn Secondary wound on a FT-37-43 core with #26 wire. [6^2 = 36 and 19^2 = 361 ---361/36 = 10! Don't forget impedance matching is done by Turns Ratio Squared!]

In the case of the 1st IF amplifier stage there is a 2 dB pad to provide a constant load for the 1st IF amp stage. It is not used on the 2nd IF Amplifier stage since the output is fed into the Product Detector. From the Crystal filter we have the same match issue from the filter (500 Ohms) to the input to the 2nd IF amp which is 50 Ohms. So we have the same 19 Turns and 6 Turns winding. Out of the 2nd If amp which is 50 Ohms it can be connected directly to the input of the product detector matching transformer. In case you haven't a clue --there is a lot of effort to have all the interfaces at 50 Ohms!

This plot is with the max voltage on Gate #2 of 4.28 Volts. If you take it to the minimum, the Gate #2 voltage would be 1.09 volts and the gain reduced by 10 dB.

That complete this evaluation of biasing gains and converting the 1st and 2nd IF amplifiers to 9.0 MHz. For the INRAD filters the Z in/out is 200 ohms and so you need a 4:1 transformer -- easily done with a 4 Turn Primary and 8 Turn Secondary on a FT-37-43 Cores using #26 wire. [4^2 = 16 and 8^2 = 64--- 64/16 = 4:1 ]

Addendum:

40 Meter Band CW Band Pass Filter

For those who are CW Ops only I have simulated the BPF that is centered more or less on 40M CW. The beauty here is that the inductors use an even number of turns. For the Capacitors use of a 50 PF Trimmer caps would work well at the four locations with C1 and C2 being fixed caps (like 130 PF) in parallel with the trimmers.

Addendum #2: In the section entitled "Other Crystal Filters" I used the term "die in the wool". I was most appropriately corrected by a pedantic observer from down under in ZL land that: 1) the term is dyed in the wool and not die (such as death) in the wool and 2) the root basis for this term is that once you "dye" wool a certain color the process is irreversible. So what you do initially --you are stuck with through to the hereafter. So I stand corrected that it should be for those homebrewer's who resist anything other than entirely homebrew filters made from discrete components to you I say you are "dye in the wool". Too bad that you settle for the status quo. Mea Culpa --Latin for I'm sorry for the error in the use of the term die versus dye.

Wednesday, December 2, 2015

Thoughts, Concerns and Considerations-Stop for a Minute to Consider Frequency Schemes.

Addendum 12/02/2015 See the video at the end of this post on Signal Handling Tests.

Part 16 was jam packed with information BUT when homebrewing amateur rigs it is always best to take two steps back and think about what ALL can happen when you make changes. I added information to Part 16 in response to inquiries about circuit information that would enable the Simpleceiver to be placed on 20 Meters.

Initially the request was for an RF amplifier stage and then I provided the Band Pass Filter constants thinking that I would receive requests for that information as well. In passing in Part 16 I mentioned you would have to change the LO frequency so that you would receive USB instead of LSB. This was done so that the same BFO frequency would be used for either band. Sometimes that is not a good solution and let us examine the WHY. Problems show up both on receive and transmit with the transmit being more serious with out of band signals when a frequency scheme has not been thoroughly evaluated!

The IF frequency of 12.096 MHz for this project was arbitrary to some degree --I had a bunch of crystals in the junk box and quickly saw that a VFO at 5.0 MHz in an additive mix (up convert)would net you the 40 Meter band. For those that have the ARRL Publication QRP Power you can see a project from W1VT, (Zack Lau) that uses that same approach for a 40M CW transceiver. So it has been done before. But this is where the professional designer (I am not one of those) would look at all mixing products and how the circuit topology can adequately handle all the possible frequency outcomes.

So in traveling down our circuit path we have the RF amp stage that was designed using LT Spice as a broader band amp versus a narrow band amp. That could or could not be a problem based on the frequency scheme so that is the first thing to evaluate. In our case the RF amp is centered on 7 MHz and drops off by 3 dB at around 14 MHz. The addendum input in Part 16 centered the RF amp on 14 MHz.

Next we have the 40M Band Pass Filter and that has a fairly good response curve where we have covered the SSB portion of the Band and all but the lower 40 kHz of the CW band are dropped off. C'mon this will be a SSB transceiver when we are done! So that part looks OK.

Now on to the mixer stage and this can be where the 1st problem is encountered. In the Simpleceiver design we are up converting the incoming frequency to the IF of 12.096 MHz. Thus our Local Oscillator PLUS the incoming signal are mixed so that the result is always 12.096 MHz. If our incoming signal was at 7.200 MHz then our LO would be tuning 4.896 MHz. If the incoming signal was at 7.15 MHz our LO would be at 4.91 MHz. Do you see a pattern here of reverse or backward tuning LO? The lower in received frequency the higher the LO. This was the same situation in the W1VT CW rig. With a digital LO the math in the software enables you to have the display read properly. You can even switch the encoder leads so that Clockwise is up in frequency or if you are left handed then CCW is up in frequency. Lots of flexibility here.

Now with this scheme the BFO frequency must be set so that you will copy Lower Sideband. An alternate way for this frequency scheme would be to set the Local Oscillator above the incoming signal so that the LO - the Incoming Signal = the IF. To receive 7.2 MHz using this approach would entail having the LO at 19.296 MHz (19.296 - 7.2 = 12.096). But when you use this approach you have a sideband inversion so the result would be USB with a BFO crystal frequency set ABOVE the filter center frequency. To receive Lower Sideband (LSB) the BFO frequency would have to be set BELOW the filter center frequency.

In our design for 40 Meters because we are not subtracting frequencies but up converting then there is no sideband inversion so that the BFO frequency ABOVE the filter Center Frequency is used for LSB and a BFO below the Center Frequency would give USB.

But in our mixer stage (the SBL-1) there are two mixing products that result (this is where there can be problems if you do not chose frequencies wisely). For our scheme we have the sum product of the incoming plus LO which results in an IF of 12.096 MHz. But we also have Incoming - the LO. Going back to 7.2 MHz our LO was at 4.896 MHz so their difference frequency is 2.304 Mhz. Our IF amplifier will easily reject that component as there is some 10 MHz of "space between these signals. Plus don't forget the Crystal filter and its ability to reject out of band signals.

Here is a plot of the 1st IF amplifier response to demonstrate this point. So our 2.304 MHz signal is about 34 dB down which is OK for our purposes. You would probably like to see this be greater than 40 dB (ideal). For such a simple design this is adequate and don't forget that the BPF ahead of the SBL-1 is not passing any 2 MHz energy and our crystal filter's ability to reject out of band signals.

The 20 Meter Case

So the above validates our choice of IF and LO frequencies for 40 Meters. While this may border on the anal retentive, let us examine what took place with the simple request to move everything to 20 Meters. That is the real danger with LT Spice --5 minutes of time and you change a few parameters and Boom a new design. But what is the impact of those changes?

So we started with the RF Amplifier and we have a plot of the response of that change to 20 Meters shown below. Our 3 dB points makes this amp work from about 8 MHz to 24 MHz. Initially we would say hey this will work for 30, 20, 15 and 15M. Did we strike Gold? Maybe not as a lot of energy across a broad spectrum is being passed on to the two section Band Pass Filter. In extreme cases it may be necessary to have a three stage BPF. So Broader is not always better!

Our next change was the 20M Band Pass Filter design. Again another 5 minutes with LT Spice and we have a new Band Pass Filter which is shown below.

This plot shows that about 1 MHz above and below the center frequency that the signal drop off is about 35 dB, which makes this acceptable for our use. Mind you the Band Pass Filter is not transceiver specific --it is simply 50 Ohms in and out and this one has a nice range.

So now on to our SBL-1 mixer stage (again this could be a problem with mixing frequencies). So now our incoming signal is at 14.2 MHz and the IF is at 12.096 MHz. We get the sum and difference frequencies at the output of the incoming and +/- the LO. Thus if the LO is 26.296 MHz and the incoming is at 14.2 MHz the difference result is 12.096 MHz --our IF. This would work well as the sum result of the Incoming plus the LO would be 40.496 MHz which is way outside any of our tuned networks. This is an important point about a term called high side mixing. By placing the LO above the incoming that neatly takes care of any sum mixing products as they are well outside the ranges of the tuned networks.

Because of high side mixing we have a sideband inversion and to receive USB the BFO has to be set above the filter Center Frequency. So the BFO frequency we used for the 40 Meter version will work on USB with the LO above the incoming frequency. There will be no need to shift the BFO frequency.

Now lets see what happens if you simply moved the LO below the incoming frequency. If we have the incoming at 14.2 MHz and our IF is at 12.096 MHz presumably we could put a 2.104 MHz LO signal into the SBL-1 and the results would be 12.096 MHz and 16.304 MHz. That 16.304 MHz component is just a few dB down from our 1st IF amplifier response curve and hopefully we have pretty good crystal filter that would fully reject that component. But more space between frequencies is always better. So low side mixing is not a good choice for the 20 Meter Band. In Part 16 where we spent 5 minutes looking at moving the LO frequency to 2.10 MHz, our further detailed look tells us to avoid that LO frequency range.

This becomes even more critical on the transmit side and improperly choosing Crystal Filter frequencies and mixing schemes could let unwanted mixing products slip through the Band Pass and Low Pass Filters. In the case where higher power is being run "traps" (tuned networks) designed for the filter IF frequency by necessity would have to be included in a low pass filter design. Take our 20 Meter example for instance 12.096 MHz will pass through a 20 Meter Low Pass Filter. But would have a tougher time with a 40 Meter filter especially if the cutoff is at 8 Mhz.

Thus having a 12.096 MHz filter was by convenience (had them in the junk box) and does enable using a 5 MHz VFO or VXO versus a digital DDS or PLL, but there are significant concerns about simply taking 5 Minutes with LT Spice and designing new bands and networks without due consideration of the possible shortcomings and unwanted signal byproducts.

A word or two here about the Simpleceiver actual hardware build. I have received an email asking me "WHY" did you do this and that which I promised I would answer those questions in a blog entry.

The hardware layout was done with some thought based on that the sections were built sequentially starting from the back end. Get the audio amp working first and then move forward through the project so that as each module is built and tested it then becomes a part of the overall test system. A recent paper I read has another view --start with the hardest module first and get that working. I guess I don't subscribe to that approach as a progressive build lets you identify immediately when a stage is not working because everything behind that stage IS working!

The use of coax cable and how did I decide when to use coax and when not to. I guess if anything is carrying RF plus if you keep all the ins and out at 50 Ohms then coax interconnect is the answer. In the case of the output from the product detector to the audio amp --that is not coax but miniature microphone cable since the output of the Product Detector is Audio. Each board that has power to the board also has a ground return to a common point supplying the minus power. When his gets done all circuit boards will be mounted in a metal box and so all circuit boards will share a common ground plane. In addition I connected a short direct ground from the IF Amp/Filter Board to the Product Detector Board.

Friday, November 27, 2015

Hey? Nobody has asked about the Crystal Filters that are being used with the Simpleceiver Project!

Addendum: 11/29/2015 Another on the air video of the Simpleceiver Addendum #2: A view of the completed Simpleceiver "al fresco"Addendum #3: 12/01/2015 Data for a 20M RF AmpAddendum #4: Signal Output Data for the AD9850Addendum #5: Band Pass Filter Data for 20 Meters

The following is the schematic for the RF Amplifier stage. Please note about the Resistor "R" and how that is made.

Addendum #3: To use this RF Amp on 20 Meters (Query from WA7RHG) simply make L1 = 8 Turns on the FT-37-43 core and make C2 = 10NF.

Addendum #4:Based on anearlier input regarding why I used the AD9850 straight into the SBL-1 without a "booster amp". I guess the simple answer is I hooked it up and it worked --so keeping thing simple. But to get full advantage of the SBL-1 it probably would be a good idea for a booster amp. Now when you measure the output of the AD9850 + Booster Amp make that measurement with a 50 Ohm load and a scope. Do not do it connected to the SBL-1.

Here are three measurements:

With the AD9850 terminated into 50 Ohms V= 324 MV PTP

With the AD9850 No Load V = 800 MV PTP

With the AD9850 connected to the SBL-1 V = 440 MV PTP

The SBL-1 is a 7 dBM device and likes to see 1.414 Volts PTP and a homebrew DBM most likely will want to see 2 Volts PTP (or 10 dBm). So a homebrew DBM will need the booster amp!

Addendum #5: Band Pass Filter Data for 20 Meters. It appears there is interest in putting the Simpleceiver on 20 Meters so here is the Band Pass Filter data. Please note if you are using the AD9850 you will need USB and so you need to take the LO plus the Offset to work 20M. Thus the VFO has to be in the 2.10 MHz range for 14.2 MHz. [NOTE in Part 17 you will see why this is not a good choice for a LO Frequency.]

The Crystal Filter

I guess this is a significant input to me as I have had no inquiries about the crystal filters being used with the radio. But just in case anyone was wondering here is some preliminary information to get you started.

First it is important to start by having three or four crystals (depending on which filter you build) to have the total frequency difference be no more than 50 Hertz across the units. To make that clear when you measure the frequencies the total difference from high to low of any of the crystals should be no more than 50 Hertz!!!! Typically I buy a dozen crystals and from that batch will find that you can usually squeeze two filters with crystals that match that hurdle. Avoid buying four crystals and simply plugging them into the circuit. If by chance you would do this and the four crystal you purchased meet that criteria -- then stop wasting your time on homebrew projects and buy a batch of lottery tickets for you are one lucky dude!

This now gets to the problem of how to measure the frequencies to within 50 Hz. I have found that many hams new to homebrewing really lack some basic test equipment and there are few alternate paths beyond such a situation. A homebrewer needs something more than a rusty screwdriver, a beat up electric drill and analog VOM that is 10% accurate.

Eventually the serious homebrewer needs an O scope, a stable RF signal source , a frequency counter and a Digital Volt Meter (DVM). Most of the modern Digital Storage Oscilloscopes have a built in frequency counter. Thus one way of measuring each crystal would be to build a test oscillator and measure the output from the oscillator and simply read the frequency on the scope. Another would be to have a frequency counter. (I happen to have both.) But something I have been recently using is my SDR Softrock transceiver along with the Power SDR software.

Typically with the Softrock, I fire up the oscillator and have a "sniffer loop" (short chunk of wire connected to the SDR antenna port) from the Softrock brought near the output of the oscillator and simply read the frequency from the software display. The desirability of this approach is that I set the Power SDR parameters to CW with the narrowest filter and then look for a peak reading on the display. Then I can note down the frequency.

But that only finds the three or four crystals that are close in frequency thus something more substantial is required to build a high quality crystal filter.There is a documented way to do this and I refer you to the following link Crystal Filter Construction from WA5BDU. Building a high quality filter can be done but must not be done in a haphazard manner. This tutorial is quite excellent and provides the formulas, equations and theory for filter construction.

Typically after I find the four crystals I sometimes think about purchasing a commercial filter. There is no reason that a 9.0 MHz commercial filter could not be installed in this circuit by modifying the 12 MHz amps to work on 9.0 MHz. With LT Spice that is not a difficult task. INRAD sells a 9.0 MHz a Four Pole filter with a Zin/out of 200 Ohms. (Model 315). 9.0 MHz filters are available also from the GQRP Club, Z in/out = 500 Ohmz. The beauty of the Arduino driving the AD9850 is that should you change the filter frequency, a few lines of code changes and you are there. The problem is somewhat more difficult using the LC VFO or a VXO. RF from the these LO's would have to be in the 2 or 16 MHz range for the 9.0 MHz IF. You get the idea.

Now for a not so high quality, not rigorously calculated, and not formally approved by the EMRFD and BITX reflectors, I usually skip the tutorial and use some values that seem to work for me. Here is exactly what I did here. I assumed a Z in/out of 150 Ohms. My listening tests have shown that the values are not too far off. But then again my goals are a bit different as I am trying to define a template for a complete project and taking a shortcut on the filter may not be the best practice; but it does get the radio to a least the point of working.

But I encourage the readers to do the rigorous calculations so you can say I know how to do it and have done it! A rigorously designed filter will undoubtedly perform better and will satisfy the need to have everything precise and tidy!

Once you have the filter built and before soldering in the circuit it would really be a good idea to test your filter. (It will be impossible to test using only the rusty screwdriver, a beat up electric drill and the 10% accurate analog VOM. Thus you will need something more. )

One of the test procedures is to terminate the filter with a 150 Ohm resistor and with the AD9850 programmed to be a signal generator as the input and placing the scope across the 150 Ohm resistor "sweep" the filter at every 50 Hz starting at 5 KHz above and extending to 5 kHz below the center frequency. Record the amplitude points and then make a plot of the points versus the frequency and you will get a shape of the filter.

If you use my method, it will be a bit ugly (maybe more than a bit ugly) but if you use WA5BDU's method you should see plots similar to what is shown in his tutorial. Alternatively you can use the AD9850 and if you have a SDR receiver repeat the same procedure and use the S Meter numbers for the plot data.

Or if you have one of those snazzy SNA's (Scalar Network Analyzer) you can run an automated test and see the plot

I want to repeat my "good enough" filters will work BUT you should do the rigorous approach as outlined in the link. Once you do that then there will never be a question of how good is your filter.

The final chapter for this part of the project will cover the receiver RF amplifier stage which is very similar to the IF amplifiers stages previously covered.

Since yesterday was Thanksgiving here in the USA, I am still in a partial eating too much Turkey coma.